The present invention relates generally to moving-coil type stereo pickup cartridges, and more particularly to a moving-coil type stereo pickup cartridge in which a vortex-shaped pattern coil, made of an electrically conductive film formed onto an electrically insulating plate, is provided in a vibrating system, and which is also contructed so as not to generate crosstalk.
In general, among the moving-coil type stereo pickup cartridges known heretofore, one type has a vibration system with structure wherein a square or cross-shaped core, around which coil wire is wound, is fixed to the rear end of a cantilever. In another known pickup cartridge of this type, two armature links are provided on a cantilever and coils are provided by winding coil wire respectively around the ends of these links.
In each of these known pickup cartridges, however, the moving-coil assembly which is fixed to the cantilever and which comprises the coil winding and the core or the coil windings and the armature links, has a large mass. Therefore, the equivalent mass of the vibration system is large, and the characteristics, particularly in the high-frequency range, are poor, and signal pickup with good characteristics over a wide band cannot be achieved. If, in order to reduce the mass, the number of winding turns of the coils is decreased, the output will drop. Consequently, it has not been possible by means of the known moving-coil type pickup cartridges to accomplish good signal pickup reproduction with high output, good signal-to-noise ratio, and flat characteristics up to the high-frequency range over a wide band.
Another difficulty encountered in the prior art has been that, since a magnetic material such as iron or permalloy has been used for the core or coil winding frame, the magnetostriction due to hysteresis and magnetic saturation is large. Still another difficulty has been that, since the coil assembly comprises coil wire wound around a winding frame, the thickness and volume of the coil structure are large. For this reason, the gap between the yoke and the pole piece in which the coil structure is interposed must be made large, whereby the magnetic conversion efficiency is poor. A further problem has been that the work of winding the coil wire around the winding frame has been laborious. Particularly, in order to obtain a high value of the above-mentioned magnetic conversion efficiency, it is necessary to reduce the thickness and volume of the coil structure, thereby to decrease the above-mentioned gap. For this purpose, a very fine wire (e.g., 10 microns in diameter) must be used for the coil wire, and this causes difficulties in the coil winding work, risk of wire breakage, and lowering of work efficiency.
Another known stereo pickup cartridge of the moving-coil type is shown in British Patent No. 939,983. This known pickup cartridge has a pair of coils, each of which is of the same size and is wound so as to be D-shaped, the turns of each coil being co-planar. Straight portions of the coils are relatively angularly displaced by 90.degree., and held between two supporting discs made of insulating material. In this pickup cartridge, however, since the mass of the coil assembly is large, the equivalent mass of the vibration system is large, and particularly the characteristics at the higher frequencies are very poor, whereby the cartridge cannot be considered practical. There is also a suggestion that these coils may be formed by printed circuits, but, with the above-described coil arrangement, reduction to practice is difficult in any case. A pickup cartridge which embodies the above concept has not yet been reduced to practice and placed on the market.
Accordingly, the applicant has previously proposed in U.S. patent application, Ser. No. 775,638, filed Mar. 8, 1977, now abandoned, a novel pickup cartridge of the moving-coil type in which the above-described difficulties have been overcome, and which has been reduced to practice.
In this previously proposed pickup cartridge, a pair of coils are formed by a thin film in a substantially hexagonal, vortex-shaped pattern on a thin glass substrate, this substrate having, for example, a height of 1 mm, a width of 2 mm, and a thickness of 50 microns. This coil plate, which is very lightweight, for example, of the order 0.25 mg, is mounted on the cantilever of the pickup cartridge. In forming the above-described coil patterns, a thin film of metal material of high electrical conductivity, such as nickel, is first formed on both surfaces of the thin insulative substrate by a process such as evaporation deposition in a vacuum. Then parts of the metal film thus deposited are removed by a process such as photo-etching in order to leave the metal film in the spirally wound pattern of the coils. A feature of this pickup cartridge is that the mass of the coil plate is very small, whereby the picking up of signals can be carried out with good characteristics up to and through the higher frequencies.
In the above-described, previously proposed moving-coil type stereo pickup cartridge, a pair of yokes form a magnetic field in a gap therebetween, in which a coil plate is interposed. Each of the yokes has a notch or cutout, the edge parts or facets of which are interconnected at right angles, making angles of plus and minus 45.degree. with respect to a line perpendicular to the record disc. However, contour lines of magnetic flux density in the space between the two facets are not assumed to be exactly parallel with the above-described facets due to the influence of leakage magnetic flux. The contour lines of magnetic flux density are curved, in particular, near the interconnection of the facets and thereby become largely nonparallel with respect to the facets.
In the case where the contour lines of magnetic flux density are not parallel with the yoke facets, which lie at angles of plus and minus 45.degree., as described above, even when the coil plate undergoes movement, for example, so as to generate output in either the left or right channel, the unexpected output signal is generated in the coil of the other channel, thus giving rise to crosstalk.
The contour lines of magnetic flux density become parallel to the facets with increasing distance from the interconnection of the facets. Therefore, if the facets of the yoke are made long, the coil plate is made relatively large, and the distance between left and right channel coils is made large, the problem of crosstalk may be relieved. However, if this structure is adopted, the reproducing characteristics in the high-frequency range deteriorate, since the mass of the coil plate is large. Consequently, the above structural organization does not have practical application.